Hormone replacement therapy and cancer risks in perimenopausal women: A retrospective cohort study using a Japanese claims database.

Breast cancer risk in women starting HRT between 45 and 49 years is lower than that in the average population; this finding might be susceptible to unmeasured factors such as early menopause among HRT recipients. Unopposed estrogen therapy accounts for considerable proportion of HRT in Japan and it increases uterine corpus cancer.